Field of the Invention
The invention relates to a method for operating a field device, wherein a measured value is assigned a current set point. A target current signal is issued depending on the current set point. An actual current signal is fed back and the target current signal is compared to the actual current signal. Furthermore, the invention relates to a field device having at least one signal outlet and at least one control component. The field device generates a measured value and the control component assigns the measured value a current set point. The control component generates a target current signal depending on the current set point. The target current signal is issued via the signal outlet and the control component feeds back an actual current signal. The control component compares the target current signal to the actual current signal.
Description of Related Art
In the field of modern process automation, field devices are often used for monitoring process variables and actuators are used for influencing processes. In the case that the field devices are measuring instruments for measuring fill levels, flow, temperatures or pH values, these signals issued from the field device correspond to the obtained measured values. Signals, which give information about the state of the field device, for example, error signals, are issued via optional special outlets.
Conventionally, the standard of 4 . . . 20 mA signals is used for signaling. Thereby, field devices are connected to a higher-level unit via 4 . . . 20 mA current loops (standard signal) or are attached to regulators and controllers having such a standard signal outlet. In addition to signal transmission, these current signals are used in 2-wire technology for energy supply of the field device. For the assignment of signals, the two limiting values 4 mA and 20 mA are usually assigned to the smallest or largest expected measured value. For the measured values between them, linear assignment is usually carried out. In the case of an error recognized in the field device, current values outside the range between 4 mA and 20 mA are issued as a standard.
It is problematic when the field device issues a value between 4 . . . 20 mA, but this value does not correspond to the set value that was assigned to the corresponding measured value.
To this end, it is known from the prior art that a current signal issued by the field device is fed back and compared to the set current value.
For example, German DE 199 30 661 A1 and corresponding U.S. Patent Application Publication 2002/0082799 A1 show a measuring transducer having a computing unit. A signal detected by a sensor is initially digitized and then fed to the computing unit. The computing unit processes the detected signal into a target value. The target value is then converted into an analog signal and issued via a signal outlet. This analog signal is fed back and transmitted again to the computing unit in digitized form. The computing unit determines a deviation between the target value and the output signal.
In the field of transmission protocols, for example, signals are superimposed with other signals or filtered and can only be compared to the originally issued signal with great effort.
One example is the widely-known HART communication. HART (Highway Addressable Remote Transducer) is a protocol for field devices having a bus address. It is one variation of digital field communication that contains many functions of field buses. In HART communication, field devices are conventionally connected using the 4 . . . 20 mA standard and joined to a higher-level unit. A digital signal is modulated onto the analog 4 . . . 20 mA signal using the FSK (Frequency Shift Keying) method. Thus, measurement, position and device data can be additionally transmitted without influencing the analog signal. Additionally, a comprehensive integration of the field device into the process control system is made possible using the HART protocol.
In the known HART standard, in which a low-frequency, analog signal is superimposed with high-frequency signals, it is common that an analog low pass filter is used for band limitation. Thus, higher frequencies of the 4 . . . 20 mA signal are filtered out. In the case that unfiltered signals are superimposed with a high-frequency signal according to the HART standard, it is difficult to differentiate higher frequencies of a broad frequency spectrum from the high-frequency superimpositions. By using an analog low pass filter, communication is simplified on the whole.
If the signal is now fed back, the superimposed signal has to possibly be removed by a digital filter. By converting the analog signal into a digital signal and due to the analog filter, time lags occur between the signals to be compared, which need to be taken into consideration when comparing the signals.